Measuring devices which make digital measured values available are being used increasingly in automation technology. In the field of numerical controls, which are used for controlling machine tools, for example, this is true especially for position-measuring devices for measuring linear or rotary movements. Position-measuring devices which generate digital measured values are referred to as absolute position-measuring devices.
Primarily serial data interfaces are used for transmitting absolute position values, since they make do with only a few data-transmission lines, and nevertheless, have high data-transmission rates. Particularly advantageous are so-called synchronous serial interfaces, which have one unidirectional or bidirectional data line and one clock line. Data packets are transmitted via the data line synchronized with a clock signal on the clock line. A multitude of standard digital interfaces have gained acceptance in automation technology. Among popular representatives for synchronous serial interfaces are the EnDat interface of Dr. Johannes Heidenhain GmbH, and the SSI interface. In addition, asynchronous serial interfaces such as Hiperface are also prevalent.
The SSI interface is described, for example, in European Published Patent Application No. 0 171 579. It is a synchronous serial data interface having one unidirectional data line and one unidirectional clock line. Position values are read out from a position-measuring device synchronized with a clock signal on the clock line.
On the other hand, European Patent No. 0 660 209 describes the fundamentals of the EnDat interface. It is likewise a synchronous serial interface which, however, besides the unidirectional clock line, has a bidirectional data line. It is thereby possible to transmit data in both directions—from the numerical control to the position-measuring device and from the position-measuring device to the numerical control. The data is transmitted in synchronization with a clock signal on the clock line, as well.
Moreover, in addition to the data which is acquired or calculated in the measuring device, there is often the desire to also transmit data, which is generated by external devices or sensors, via the existing interface connection, e.g., between a position-measuring device and a numerical control.
For example, German Patent No. 10 2006 041 056 describes providing a rotary encoder with additional interfaces to which external sensors may be connected, the sensor data being able to be processed in the encoder and transmitted via a bus interface to a control.
German Published Patent Application No. 103 06 231 describes an intermediate assembly, to which, on one hand, peripheral equipment, e.g., sensors, may be connected, and which, on the other hand, may be connected by a communication interface to a position-measuring device. The position-measuring device may, in turn, process the sensor data and/or output it via an interface connection in the direction of a numerical control.
Both variants require at least one additional interface on the rotary encoder, which moreover, must also be accessible in the incorporated state.